43e.1 Arthropods, diverse and ancient, are a challenging group to classify.
Arthropods comprise about 85% of metazoan species and have achieved a remarkable structural and taxonomic diversity.
In their quest to reveal more about metazoan phylogeny, scientists are beginning to explore why arthropods are so diverse and to accurately construct the arthropod phylogeny.
In the 1930s, Robert Snodgrass proposed a classification of arthropods that is still used today.
Snodgrass' classification implies that arthropods are monophyletic, myriapods and hexapods are sister group (Uniramia), and the Uniramia and Crustacea form a sister group, the Mandibulata.
Recent work focusing on molecular phylogenetic analyses, homeobox genes, developmental biology, and new fossils finds is starting to challenge Snodgrass' classification.
43e.2 Four types of studies are revolutionizing arthropod classification.
- Morphological studies of arthropod phylogenetics all suggest that arthropods are monophyletic.
- Several features of arthropod nervous systems are more similar between insects and crustaceans than between insects and myriapods.
- Molecular phylogenetic studies of various arthropod gene sequences and the arrangement of mitochondrial genes have yielded similar results.
- The molecular phylogenetic studies imply that arthropods are monophyletic, crustaceans and insects alone constitute a sister group (making the terms Mandibulata and Tracheata obsolete), and crustaceans may be paraphyletic.
- Thus, molecular phylogenetics suggest that morphological similarities between insects and myriapods are not the result of common ancestry.
- Hox (homeobox) genes control developmental pathways, such as the formation of body axes and different types of limbs and the location of the different types of limbs.
- Hox genes are very ancient and remarkably conserved. By looking at the patterns of expression in animals, biologists can deduce evolutionary relationships.
- One of the hox genes, Distal-less is expressed in the jaws of myriapods but is not expressed in the jaws of crustaceans and insects, again suggesting a close relationship between the latter two groups.
- In the new fossil discoveries, crustaceans are more numerous and morphologically diverse than trilobites, suggesting that Crustacea likely evolved before the trilobites.
- Cladistical analyses of fossil arthropod data place Crustacea at the base of the arthropod tree and indicate paraphyly in the Crustacea.
43e.3 Taxonomists have also been reevaluating related groups.
- Molecular phylogenetic and morphological analyses suggest that pentastomids, respiratory parasites of vertebrates, are modified crustaceans.
- Onychophorans are present in many of the early fossil formations.
43e.4 Current views of arthropod diversity and classification.
- The abundance and diversity of arthropods now is not surprising due to the ancient age of arthropods and arthropod diversification.
- The small sizes of many arthropods allowed the invasion of many types of niches and radiation within these niches. In addition, as the first flying animals, arthropods had many other niches open to them.
- Segmentation and the diversity, age, and flexibility of Hox genes have facilitated the evolution of a diverse array of arthropod body types.
- Coevolution with plants and algae has been a strong force driving the radiation of insects and crustaceans.
- All types of studies indicate that arthropods are monophyletic.
- In contrast to traditional morphological studies, new morphological, molecular, and developmental studies suggest that insects are more closely related to crustaceans than to myriapods.
- Fossil studies indicate that crustaceans are the oldest arthropods and may have given rise to all other groups of arthropods. Molecular and fossil studies reveal that crustaceans may be a paraphyletic group.
- Isopods are a likely ancestor of insects due to many morphological similarities between the two groups, especially among elements of the arthropod nervous system.